As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has been also sharply increased as an energy source for the mobile devices. One of the secondary batteries is a lithium secondary battery having high energy density and discharge voltage, on which much research has been carried out and which is now commercially and widely used.
Based on its external shape, the lithium secondary battery is generally classified as a cylindrical battery, a rectangular battery, or a pouch-shaped battery. Based on its electrolyte, the lithium secondary battery is classified as a lithium-ion battery or a lithium-ion polymer battery. As the mobile devices have been miniaturized, the demand of the rectangular battery and the pouch-shaped battery, which have a relatively small thickness, has increased.
Furthermore, based on how the secondary battery is mounted to a case, a battery pack is generally classified as a hard battery pack or an inner battery pack. A typical example of the hard battery pack is shown in FIG. 1. Referring to FIG. 1, the hard battery pack 10 forms a part of the external appearance of an external device 12, to which the hard battery pack 10 is mounted. Consequently, the hard battery pack 10 has an advantage in that the hard battery pack can be easily mounted to the external device 12 when in use. However, it is required to design a case (housing) 11 based on kinds of external devices while a battery cell (not shown) is mounted in the case. As a result, the manufacturing costs of the hard battery pack 10 are high, and, furthermore, the hard battery pack 10 has low compatibility with the external devices.
On the other hand, as shown in FIG. 2, the inner battery pack 20 is mounted in an external device, and is then hidden by a cover, which forms a part of the external device. As a result, although it is relatively troublesome to mount the inner battery pack 20 in the external device, the inner battery pack 20 has advantages in that it is easy to design the inner battery pack, the manufacturing costs of the inner battery pack are low, and the inner battery pack has high compatibility with the external devices.
The details of an inner battery pack including a rectangular battery cell will be described with reference to FIGS. 3 and 4. Referring to these drawings, the inner battery pack 20 comprises: a rectangular battery cell 21 having a cathode terminal formed at one side thereof and an anode terminal formed at the other side thereof; a positive temperature coefficient (PTC) element 22 connected to one of the two electrode terminals of the battery cell 21 for primarily protecting the battery from overcurrent, overdischarge, and overcharge; a protection circuit unit 24 connected to the PTC element 22 side electrode terminal (the cathode terminal or the anode terminal) through a nickel plate 23 and connected to the other electrode terminal through a nickel plate 27 for secondarily protecting the battery, the protection circuit unit 24 being provided at the outside thereof with external input and output terminals, through which the protection circuit unit 24 is connected to corresponding external devices (not shown); an upper case 25 and a lower case 26 for surrounding the battery cell 21, the PTC element 22, and the protection circuit unit 24.
Between the side of the battery cell 21 and the nickel plate 23 and between the protection circuit unit 24 and the nickel plate 27 are disposed insulation sheets 28, which prevent short circuits due to unnecessary contact between the nickel plates 23 and the battery cell 21 or between the nickel plates 27 and the protection circuit unit 24.
Also, a double-sided adhesive tape 29 is disposed between the battery cell 21 and the lower case 26 such that the battery case 21 can be tightly fixed to the bottom of the lower case 26 by means of the double-sided adhesive tape 29. Consequently, when the battery cell 21 is received in the upper and lower cases 25 and 26, the battery cell 21 can be stably fixed in the upper and lower cases 25 and 26.
After the upper case 25 is coupled to the lower case 26, in which the battery cell 21 is received, the coupled upper and lower cases 25 and 26 may be surrounded by a packing label 30, which further increases the coupling force between the upper and lower cases 25 and 26 and prevents external foreign matters from being introduced into the coupling regions between the upper and lower cases or into the upper and lower cases.
FIG. 5 is a perspective view illustrating a conventional inner battery pack including a pouch-shaped battery cell, FIG. 6 is an exploded perspective view of the battery pack shown in FIG. 5, and FIG. 7 is a perspective view of the battery pack shown in FIG. 5, which is partially assembled.
Referring to these drawings, the battery pack 50 includes a pouch-shaped battery cell 51 having an electrode assembly, which consists of cathodes, anodes, and separators, mounted therein together with an electrolyte in a sealed state, a lower case 52 having an inner space for receiving the battery pack 51, and an upper case 53 coupled to the lower case 52, in which the battery cell 51 is received, for sealing the battery cell 51. Also, double-sided adhesive tapes 54 are disposed between the battery cell 51 and the upper and lower cases 53 and 52 such that the battery cell 51 can be stably fixed in the inner space defined by the cases 52 and 53.
After the upper case 53 is coupled to the lower case 52, in which the battery cell 51 is received, the coupled upper and lower cases 53 and 52 are surrounded by a packing label 40, in the same manner as the assembly process of the battery pack 20 shown in FIGS. 2 to 4, such that the coupling force between the upper and lower cases 53 and 52 is increased, and external foreign matters are prevented from being introduced into the coupling regions between the upper and lower cases or into the upper and lower cases.
In the battery pack 20 of FIG. 2 and the battery pack 50 of FIG. 5, the upper and lower cases are made of a plastic material, such as polycarbonate (PC) or polyacrylonitrile-butadiene-styrene (ABS), and the upper and lower cases are securely coupled to each other by an ultrasonic welding method. The ultrasonic welding method is a method of welding two surfaces to be attached using frictional heat generated by vibrations of high frequency, for example, 20,000 Hz.
The details of the coupling between the lower case and the upper case by the ultrasonic welding method will be described with reference to FIGS. 8 to 11. FIG. 8 is a plan view illustrating the upper case 53 mounted on the lower case 52, and FIG. 9 is a vertical sectional view (a sectional view taken along line A-A of FIG. 8) of the coupled the upper and lower cases 53 and 52. While the upper case 53 is mounted on the lower case 52, opposite ends of the upper case 53 are in contact with opposite ends of the lower case 52. FIG. 10 is a partially enlarged view illustrating the contact part B of FIG. 9 before ultrasonic welding, and FIG. 11 is a partially enlarged view illustrating the contact part B of FIG. 9 after ultrasonic welding. As shown in FIG. 10, a wedge-shaped welding ridge 61 is formed at the end of the upper case 53, and a welding surface 71 is formed at the end of the lower case 52 such that the welding surface 71 can be brought into contact with the welding ridge 61. When high-frequency vibrations are applied to the welding ridge 61 and the welding surface 71, the contact surface between the welding ridge 61 and the welding surface 71 is welded, and therefore, the welding ridge 61 and the welding surface 71 are attached to each other.
However, as the demand of small-sized battery packs having further decreased thicknesses has increased, the thicknesses of the lower case 52 and the upper case 53 have been reduced down to 0.3 to 0.35 mm in recent years. As a result, it is difficult to manufacture the lower case and the upper case by die casting and injection molding. Furthermore, as the sizes of the welding ridge 61 and the welding surface 71 are decreased, the welding strength is reduced, and therefore, poor welding rate is increased.
Also, when the upper case having a small thickness and the lower case having a small thickness are welded to each other by the ultrasonic welding, the coupling force between the upper case and the lower case is relatively small. Consequently, it is required that a film-shaped wrapping label be applied to the outer surface of the battery pack so as to secure a coupling force necessary to be used as the battery pack and to prevent foreign matters from being introduced into the coupling regions between the upper and lower cases or into the upper and lower cases.
For this reason, a battery pack that has an adequate strength against the external impacts even using small-thickness cases and that can be easily manufactured with reduced manufacturing costs is highly required.